The origin of (47171) Lempo-like Kuiper belt triple systems during binary–binary interactions

Abstract

ABSTRACT The trans-Neptunian triple Lempo–Paha–Hiisi is composed of a tight inner binary with components of similar size and an outer companion about half their size orbiting 10 times further away. Large trans-Neptunian objects like Pluto also have multiple small moons, but Lempo’s structure is unique in the Solar system, and the place and timing of its origin is still a subject of debate. We propose a new formation mechanism able to form a large number of systems like Lempo–Paha–Hiisi, which involves binary–binary close encounters in the primordial planetesimal disc at 30–40 au. Some of these systems were then implanted in different populations of the trans-Neptunian region during Neptune’s outward migration. Our results strongly support that the 4:7 resonant multiple object Manwë–Thorondor was once a triple system similar to Lempo–Paha–Hiisi, but the orbit of the inner binary evolved by tides, becoming a contact binary. As with Lempo–Paha–Hiisi, it should have originated in the planetesimal disc below 30–40 au. Triple systems like Lempo–Paha–Hiisi or Manwë–Thorondor could not have formed in situ and the existence of this kind of system is not expected in the cold classical Kuiper belt.